Instructions for using the Audacity Noise removal effect in the Audacity online manual here. The instructions are straightforward and clearly written.

Note what it says about selecting a region to get a noise profile, then selecting the region you want to remove the noise from. If it's the whole track, then do as Mick Pearce says. I usually press the "home" key on my laptop which does the same thing.

Note what it says about mains hum and using a notch filter first. It is quite possible the noise the noise from the hard drive the OP describes is at mains frequency (50 or 60 Hz, depending where you are) or possibly, a multiple of mains frequency.

It's also worth testing the effect of the input volume. On my Linux system in the sound settings there is a slider for input volume and I find that if that's set too high, the noise starts to dominate and trying to remove it has too much of an adverse effect on the signal. I find I need to keep the input level down quite a lot. It may reduce the signal level on your recording but the effect on the noise seems to be much greater. I suspect there is an internal preamp and that adds noise to the mic signal.